Chemistry Department, Poona University, Pune , India Received 23 April 1990, accepted 24 July 1990

Transcription

1 Indian J. Phys. MB (5), (1990) Study f slvent effect n electrnic absrptin spectra f sme chlranilines. Part II* : Excited state diple mments and specific slute-slvent interactin energies based n Bakhshiev s expressin fr slvatchrmic frequency shifts L S Prabhu M irashit and R S Varkhede Chemistry Department, Pna University, Pune , India Received 23 April 1990, accepted 24 July 1990 The excited state diple mments (fig) and specific slute-slvent interactin energies ( «) f sme chlranilines discussed in a recent paper (Prabhu Mirashi and Varkhede 1989) were estimated by emplying McRae's expressin (McRae 1957) fr slvent induced frequency shift in electrnic absrp tin spectra. It is wellknwn that the derivatin f McRae's expressin invlves several simplifying apprximatins due t a large number f unevaluated parameters appearing in the perturbatin calculatin. It is bserved that the expressin fr J v deduced by Bakhshiev (1961, 1962) by emplying a mre descriptive and classical electrstatic apprach invlves much smaller number f apprximatins and n unevaluated parameters. Bakhshiev himself r any ther grups have nt applied this particular equatin but have always used it in cmbinatin w ith a similar ne fr the shifts in flurescence bands. Recently we have illustrated (Prabhu Mirashi and Kunte 1989, 1990) the use f Bakhshiev's expressin fr Jv jp absrptin spectra alne fr estimatin f /igs and E,s f sme substituted benzenes. Therefre fr the sake f examining general applicability f the latter apprach, the ftgs and $ f chlranilines have been recalculated n the basis f Bakhshiev's expressin and cmpared w ith the earlier values based n McRae's expressin. Bakhshiev's (1961, 1962) expressin fr slvatchrmic frequency shift f an electrnic absrptin band is cnveniently expressible in fhe frm : ^ + C xf(d,n ) + C af(n) -h C, f (n ) C * r (n ) cm" ^ where the varius functins and parameters are defined as *Part I ; Ref.: Prabhu Mirashi and Varkhede (1989). fpresent Address : Chemistry Department, Ga University, Bambli, Ga , India ( 1)

2 408 L S Prbhu Mlrdshl and R S Varkhede f(n) = ( 2 n * + 1 ) * ( n * - 1 ) /( n " + 2 )», f'(n) = (n«~ 1 )/( 2 n " + 1 ), f"(n) = ( n «- 1 )/(n* + 2 ) and RD, n) = [(2 n«+ 2 )/(n» + 2 )]» x [ ( D - l ) / ( D + 2 ) - ( n * - 1)/(n» + 2 )] e)ihcal and C,= r ( ^ j- p, )/hc» (2 ) Here n and D are the refractive index and dielectric cnstant f a slvent ; Hg and fie the grund and excited state diple mments f a slute mlecule w ith e being the angle between their directins ; C., and are the measures f dynamic and dispersin shifts invlving plarizabilities, inizatin ptentials, etc. f the slute and slvent mlecules in their grund and excited states ; h the Planck cnstant, c the spaed f light and a the Onsager cavity radius. A systematic and step-wise prcedure illustrating the applicatin f eq. ( 1 ) has been explained in ur recent papers (Prabhu Mirashi and Kunte 1989, 1990). Accrdingly, in the case f plar slutes in saturated hydrcarbn (HC) slvens (D - n =»f(d, n ) - 0 ), eq. ( 1 ) can be rewritten in the frm j s that the slpe f the linear graph f the L.H.S. f eq. (3) as a functin ^ f(n) tgether w ith the knwn values f /tg (McClellan 1974) and thse df estimated frm mlecular dimensins (Suttn et l 1958, 1965 : Pauling 1960, Parkanyi et al 1986, Aarn et al 1987, Prabhu Mirashi and Varkhede 1989) lead t the value f /x«. The values f C 9 and C* parameters required in eq. (3) abve were used frm ur previus wrk (Prabhu Mirashi and Kunte 1989). The specific slute-slvent interactin energies (Ea) fr a slute w ith different nnhydrcarbn (nnhc) slvents were estimated frm the difference between the experimentally measured frequency (^scba)) and that calculated (^,r.i)) n the assumptin f nly universal interactins as implied in eq. ( 1 ), by using the expressin (Prabhu Mirashi and cwrkers 1989, 1990). I, ( ^, «i j - 5.(ub,)) kj m l-" (4) The experimental methd and materials used in the present wrk were the same as in ur previus wrk (Prabhu Mirashi and Varkhede 1989). As mentined earlier (Prabhu Mirashi and Kunte 1989). Bakhshiev's expressin fr dj is frmally very sim ilar t that f McRae (1957). The sum f the dynamic and dispersin shift parameters viz., C g ( cm "") and C g ( = cm"") fr the Lt transitin f benzene is in clse agreement w ith the value f the similar (A + B ) parameter, viz cm"", appearing in McRae's equatin (Prabhu Mirashi and Vakhede 1989). Als, the plts f t, - [ C 9 f'(n)-l-c g f'(n)] vs f(n) crrespnding t measurements in HC slvents exhibit gd linearity w ith crrelatin

3 Study f slvent effect n electrnic absrptin spectra etc 4 cefficient ~ 0.9. The m s are all larger than the crrespnding iigs. The u s fr the three mnchlranilines are fund t vary in the rd er: > /u«(m) > fie() while thse fr the di- and trichlranilines (DCA and TCA) are bserved t exhibit the rder: Mt(m, m'dca) > /i«(, p, 'TCA > /i«(, 'DCA). These trends parallel t thse exhibit by the respective ngs. All these bservatins and trends are similar t thse bserved in ur previus wrk (Prabhu Mirashi and N/arkhede 1990) based n McRae's equatin. The btained n the basis f Bakhshiev's equatin are all systematically lwer than thse based n the applicatin f McRae's equatin. Since single electrn excitatins are nt expected t bring abut drastic change in electrn density distributin within a mlecules; and als in view f lesser number f apprximatins and absence f undetermined parameters in Bakhshiev's expressin ; the present /tes appear mre reliable. While deducing the 5g(c,i) values by emplying eq. (1), the required vapur phase frequencies (F ) are frequently nt available. Therefre, in view f eq. (4), the intercept f the graph f i'f-[c a r (n ) + C 4f"(n)] vs f(n) crrespnding t spectral measurements in HC slvents (i.e. the value f Uq crrespnding t f(n)=0=»vaccum) is cnsidered as an apprximate measure f C. The Vq values fr benzene, chlrbenzene, aniline, m-nitraniline and p-nitraniline s btained, viz , 39045, , and cm-^ respectively are In favurable agreement with the reprted values, viz , 39260, 36200, and cm'^ respectively (Kamlet and Ungnade 1960, 1963; Millefiri 1977). Therefre, the present apprximatin f ~ i',, can be cnsidered as reasnable. Further the cefficient f the functin f(d, n) in eq. (1), viz. C i-2 ^ (/ia -fi«cs e)/hc invlves the angle between the directins f the fi and fie vectrs. Hwever, fr majrity f armatic mlecules, 6 is knwn t be quite small s that the fig and fie vectrs can be assumed t be cllinear (Bakhshiev et al 1969). This assumptin is cnsistent with the results f ther wrkers (Grass et al 1976, Millefiri et al 1977). The values fr varius slute-slvent systems estimated by using eq. (4) n the assumptin f and while btaining the required values are listed in the Table 1. These E, values are als f the same rder f magnitude as thse btained n the basis f McRae's equatin in the previus wrk (Prabhu Mirashi and Varkhede ): and, like the m,s the present E, values are als generally smaller than the previus nes. The difference in the tw sets f E -values can be attributed t thse in the estimates f values based n Bakhshiev's and McRae's equatins. Hwever, the general trends exhibited by bth the sets f E, values are quite similar. It is thus bserved that the results based n Bakhshiev's expressin are essentially similar and quite cmparable with

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5 Study f sl\^ent effect n electrnic absrptin spectra etc 411 thse based n McRae's expressin. These results illustrate general validity f Bakhshiev's expressin fr studying the slute-slvent interactins frm frequency shifts in electrnic absrptin spectra. References Aarn J J, Gaya M D, Parkanyi C, Ch N S and Vn Szentpaly L 1987 j. M/ec. Struct, (Thechem ) Bakhshiev N G 1961 Opt. Spectrsc, Opt. Spectrsc ; Bakhshiev N G, Knyazhanskii M I, Osipv 0 A and Saidv G V 1969 Russian Chem, Revs Grass D, Pasne S, Gandlf C and Buemi G 1976 Tetrahedrn 32 ^^05 Kamlet M J and Ungnade H E 1960 Organic Electrnic Spectral D ata Vl 1 (New Yrk: Interscience) Ib id Vl 4 McClellan A L 1974 Tables f Experim ental Diple Mments Vl 2 (Califrnia : Rahara Enterprises) McRae E G 1957 j. Phys. Chem Millefiri S, Favini G, Millefiri A and Grass D 1977 Spectrchim. Acta 33A 21 Parkanyi C, Oruganti S R, Abdelhamid A 0, Vn Szentpaly U Ngm B and Aarn J J 1986 J. M/ec. Struct. fthechem ) Pauling L 1960 The Nature f T/ie Chemical Bnd 3rd ed (Ithaca, New Yrk : Crnell University Press) Ch 7 Prabhu Mirashi L S and Varkhede R S 1989 Indian J. Phys. 63B 454 Prabhu Mirashi L S and Kunte 5 S 1989 Spectrchim. Acta 45A India n J. Chem. 29A 215 Suttn L E, Dnahaue J and Bwen H J M 1958 and 1965 Tables f Interatm ic D istances and Cnfiguratins in m lecules and Ins Special Publicatin Ns 11 and 18 (Lndn: The Chemical Sciety)